1. Field of the Invention
This invention relates to a welding robot and, more specifically, to a system for converting welding conditions for a welding robot controlled by a numerical control unit and a welding machine controlled by the numerical control unit and equipped with a torch grasped by the robot. More particularly, the invention relates to a system for converting welding voltage and welding current.
2. Description of the Related Art
Arc welding wherein an electric arc is continuously generated and the heat thereof utilized to join two metals has long been known. Recently, with the advancement of numerically controlled robots, welding robots in which such a welding operation is performed by the robot have come into practical use. Specifically, a welding robot is adapted to grasp the torch of a welding machine, move a hand of the robot in response to a command from a numerical control unit to move the torch along a welding path and perform a welding operation while the welding state is controlled by the numerical control unit. Before starting welding with a welding robot of this kind, the operator performs a so-called teaching operation in which the welding robot is actuated manually to move the torch along a welding path, during which time the numerical control unit memorizes the path. Then, such welding conditions as the commanded value of welding voltage, the commanded value of welding current, the commanded value of preflow time and the commanded value of postflow time are fed into the numerical control unit from an operator's panel and set in or loaded into the numerical control unit.
In the conventional welding robot of the above-described type, the welding voltage and welding current commands that the welding machine receives from the numerical control unit are digital command values produced by the numerical control unit and converted into analog values by a digital/analog converter (hereinafter referred to as a D/A converter). These analog values are applied to the welding machine. FIG. 1 is a view showing how the input signal to the D/A converter is related to welding voltage applied as a welding condition input value to the numerical control unit (NC). For example, when a commanded welding value representing 50 volts is to be input to the D/A converter to produce an analog value representing the 50 volts, the graph converts the command value into a digital representation of 100 and the D/A converter then converts the 100 into an output voltage of 5 volts. The welding voltage applied to the numerical control unit (NC) is plotted along the horizontal axis, and the input to the D/A converter is plotted along the vertical axis. Ordinarily, the horizontal axis is divided into a scale of from 1 to 100 and the maximum value of welding voltage taken along the vertical axis is 200 V, which is determined by the three-phase commercial supply voltage used. If the digital commanded value applied to the numerical control unit (NC) ranges from, say, 0 to 200 V, the relationship of the digital commanded value applied to the D/A converter input is 1:1, so that if 100 V is specified by the numerical control unit (NC), a 100 V output is delivered to the D/A converter. In other words, EQU PARAMETER ANOCl=200/V (D/A converter input value)
from which we have EQU PARAMETER ANOCl=200/200=1
so that the input signal to the D/A converter has a 1:1 relationship with respect to the commanded value applied to the numerical control unit.
If the digital commanded value applied to the numerical control unit ranges from, say, 0 to 50 V, we have EQU PARAMETER ANOCl=200/50=4
Thus, the parameter is an integer.
However, if the commanded value applied to the numerical control unit ranges from, e.g., 0 to 60 V, the parameter ANOCl is not an integer and the conversion process is very troublesome.
In addition, as shown in FIG. 1 by the dashed line, the digital/analog conversion characteristic cannot be provided with an offset characteristic.